With my Valve Index, when the headset or controllers go near the boundaries of the play space (approx. 1.5ft from the edge for a 9x9ft area), SteamVR will overlay a bright cyan wireframe of what the cameras see on top of the game. For me, this works incredibly well and I've yet to hit my controllers on anything (well, except for clapping the controllers together).

I'd be curious to know if others disable this feature intentionally or if this method of warning the user isn't as effective with everyone (I am very easily distracted by anything popping up). I could see it being annoying for a smaller play space.

(Also, does Oculus/non-SteamVR have an equivalent? I assume it must)